It is generally desirable to employ a base in a heat developable light-sensitive material in order to accelerate development by heat. In order to increase stability of the light-sensitive material, it is necessary to employ the base in the form of a precursor. The term "precursor" as used herein means a compound which thermally decomposes and releases a basic component by heating. In order to employ such a precursor in practical use, it is necessary that the precursor have two opposite properties, i.e., stability at normal temperature (about 0.degree. C. to 35.degree. C.) and rapid decomposability at the time of heating.
Basic precursors which have been heretofore known include, for example, a urea as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554, a method using urea or urea and an ammonium salt of a weak acid as described in Japanese Patent Publication No. 1699/65), a method using hexamethylenetetramine or semicarbazide (as described in U.S. Pat. No. 3,157,503), a method using a triazine compound and a carboxylic acid (as described in U.S. Pat. No. 3,493,374), a dicyandiamide derivative (as described in U.S. Pat. No. 3,271,155), an N-sulfonyl urea (as described in U.S. Pat. No. 3,420,665), an aminimido (as described in Research Disclosure, RD-15776 (1977)), and a salt of a thermally decomposable acid represented by trichloroacetate (as described in British Pat. No. 998,949), etc.
However, image forming materials using these base precursors have essentially serious defects. Specifically, these compounds cannot fulfill the above described indispensable conditions, i.e., good stability during preservation at normal temperature and rapid decomposition at the time of development processing, and cause problems in that a high image density cannot be obtained or the S/N ratio of image is seriously decreased due to release of the base during preservation.